1. Field of the invention
This invention relates to a method and an apparatus for high-speed and high-precision inspection of surface conditions of various articles having not only flat surfaces of metal or glass sheets or the like but also undulating or stepped surfaces on components of machines and electrical appliances and instruments, detecting locations, sizes and nature of flaws or defects if any
2. Description of the Prior Art
The inspection of flaws or defects on the surfaces of materials and products is essential in various industrial fields, for example, in the fields of automobiles, electrical appliances and so forth. In the surface inspection, in addition to the locations and sizes of surface flaws or defects, it is normally required to detect the kind or nature of surface flaw or defects
To meet such a requirement, it has been the usual practice for the conventional surface inspection machines to resort to the so-called flying spot method using a laser beam in a light-spot scanning system. In the flying spot method, a light beam from a laser source is spotted on and scanned along a surface of specimen, inspecting the surface condition by means of a detection system which receives reflected and diffracted light from the surface under inspection. Existing detection systems are largely classified into multiple photocell type, diffraction pattern projection type, mirror convergence type and diffuser convergence type. In any case, the inspection is restricted to plano surfaces such as surfaces of flat glass sheets, metal sheets and film-like materials, and not applicable to articles which contain ups and downs or undulations on the surfaces thereof More specifically, in case of a specimen with stepped surface portions, the surface level for inspection varies from one position to another, so that an obliquely incident scanning light beam is reflected at different positions (levels) on the surface of a specimen, the reflected light often failing to enter the detection system to form an image therein.
Even in a case where the reflected light is anyhow led into the optical detection system, it is difficult to detect an accurate diffraction pattern of reflected and diffracted light from a specimen surface, which is essential for accurate judgement of the nature of a surface flaw or defect.
FIG. 4 shows by way of example the construction of a conventional surface inspection apparatus based on the multiple photocell method, in which a light beam from a laser generator 31 is projected toward a vibrating mirror 33 through a collimator lens 32, thereby changing the direction of the irradiating light beam at high speed to scan the surface of a specimen 34 from an oblique direction. On the part of the photometric detection system, a couple of photocells 35 and 36 are provided side by side, one for reflected light and the other for diffracted light, the output of these photocells being sent to an analogue arithmetic unit 39 through amplifiers 37 and 38, respectively.
The photometric detection system of this sort is capable of determining the amount of diffracted light but due to its construction incapable of determining the direction of the diffraction pattern which is necessary for judging the nature of a surface flaw.
Further, as shown in FIG. 5, in the diffraction pattern projection method, a light beam from a laser beam generator 41 is projected on a specimen 42, and the image of a diffraction pattern which appears on a screen 43 is taken by a camera or other suitable image pick-up means for judgement of the nature of surface flaws. In this case, however, the measurement gives information of a particular point, and the specimen or optical system has to be moved for two-dimensional scanning in order to obtain information on a certain area of a surface, requiring a great deal of time for inspection.
Thus, the conventional technologies in surface defect detection do not permit three-dimensional measurement of a specimen simultaneously with the detection of a diffraction pattern, making it difficult to judge the nature of flows or defects on the surface of specimen accurately at high speed especially in a case where the specimen has an undulating or stepped surface.